This invention relates to an improvement for producing crystals of malic acid having a uniform high purity in the production of malic acid by using maleic acid and/or fumaric acid as a raw material. Malic acid is in great demand as food-additives such as flavoring agent, acid-tasting agent, etc. Therefore, it is required to produce malic acid crystals of a uniform high purity.
The production of malic acid (in a D,L form) by heating an aqueous solution of maleic acid and/or fumaric acid is well known.
In the known process, maleic acid and/or fumaric acid is dissolved in water in an amount of 30-130 g per 100 ml of water. The solution is heated in an autoclave at 150.degree.-250.degree. C for about 3-10 hours to form malic acid. The resulting reaction mixture contains trace to about 3% of maleic acid, about 30-70% of fumaric acid and about 30-70% of the desired malic acid (percentages are based on the total amount of acids). As the first step for isolating malic acid from the reaction mixture, the mixture is cooled at 10.degree.-40.degree. C, usually 20.degree.-30.degree. C to precipitate and remove fumaric acid having a low solubility. The resulting mother liquor still contains 1-3%, usually about 1.6% of fumaric acid. It is obviously disadvantageous to further cool the mother liquor to remove the remaining fumaric acid, since a large amount of malic acid is precipitated together with fumaric acid in such a cooling. Accordingly, there has been no choice in a conventional process for the isolation of malic acid but to directly subject the mother liquor to decolorization and concentration followed by the crystallization of malic acid. Sometimes, the unreacted maleic acid is removed from the mother liquor prior to the concentration of the mother liquor, but usually the maleic acid is retained in the mother liquor under properly controlled conditions while malic acid is crystallized in and isolated from the mother liquor. The isolated crystals of malic acid are dried for finishing.
The crystallization of malic acid is effected by cooling the concentrated mother liquor containing malic acid usually in a ratio of 65-80% based on the total amount of the acids. However, according to the conventional process, it is very difficult to stabilize the formation of malic acid crystals of a high purity and proper size. When, for example, the concentrated mother liquor obtained after separating fumaric acid crystals from the reaction mixture, is left to stand for about 5 hours and then cooled with water for about 20 hours, the concentrate is converted to a paste. It is difficult to isolate malic acid crystals from the paste. Malic acid crystals separated from the paste not only have a low purity but also are irregular in size. Among the crystals there are contained crystals of a microscopic size.
The present inventors have made various studies on the isolation of crystals of a desired compound from such an aqueous solution of organic acids as the above-mentioned concentrate, from which it has been difficult to separate the crystals favorable in properties. As the result, it has been found that crystals having a high purity are readily obtained by: as the second step, cooling the aqueous solution of organic acids as mentioned above (for a specific example, the aforementioned aqueous solution of malic acid obtained after the first step of the separation of fumaric acid and the concentration of the mother liquor) at a temperature within a given range in order to effect the crystallization of the remaining fumaric acid, and separating the resulting crystals with a comparatively small loss of malic acid; and, as the third step, cooling the resulting residue to effect the crystallization of malic acid and separating the resulting malic acid crystals. The present invention has been accomplished on the basis of this finding.
Particularly, it has been found that malic acid crystals, which are uniform and filtered off, are produced by cooling as the second step the aqueous solution of malic acid having such a high concentration as 55-90%, usually 65-80% obtained in accordance with the aforementioned manner, at 21.degree.-70.degree. C, exactly, at a temperature within the range shown as a shade in FIG. 2; and cooling as the third step the filtrate obtained after separating the resulting crystals in order to crystallize malic acid. In FIG. 2, the cooling in the shaded area can be expressed as cooling the concentrate having a concentration of 65-80% by weight at a temperature of about 20.degree.-45.degree. C at the lower weight percent and about 65.degree.-75.degree. C at the higher weight percent. The cooling at intermediate concentrations is carried out generally on a straight line basis between the lower temperature ranges and slightly under the upper temperature ranges on a straight basis.